Pickling process



Sept. 30, 1947. w, HUDSON 2,428,221

PICKLING PROCESS Filed Feb. 6, 1942 2 Sheets-Sheet 1 AbfV TOR lmzmATTORNEYS Sept. 30, 1947- J. w. HUDSON PICKLING PROCESS Filed Feb. 6,1942.

2 Sheets-Shut 2 INVENT 3 w 9 r MiM ZM ATTORNEY Patented Sept. 30, 1947PICKLING PROCESS James W. Hudson, Upper Darby, Pa., assignor to WalkerBrothers, Conshohocken, Pa., a corporation of Pennsylvania ApplicationFebruary 6, 1942, Serial No. 429,739

3 Claims.

This invention relates to the pickling of iron and steel objects toremove oxides formed by atmospheric corrosion and hot processing. Moreparticularly, the invention is concerned with a novel method forpickling which offers many advantages over the methods now commonlyused, and with apparatus by which the new method can be carried onefiiciently.

In the manufacture of iron and steel articles, it is necessary in manyinstances to clean the articles thoroughly as a preliminary tosubsequent operations, such as galvanizing, electroplating, etc., andsuch cleaning ordinarily involves operations of two different types. Thefirst cleaning operation serves to remove the oils, greases, and otherorganic contaminants employed in machine operations, such as tapping andthreading, and the second is employed for the removal of rust and millscale.

Various methods are now available for removal of greases and oils and,for that purpose, the objects may be treated with alkalis or $01- vents,or subjected to electrolytic cleaning. While these various processeshave objectionable features, they are in common use and the-selection ofa particular process depends on the nature of the articles being treatedand economic and other considerations.

Pickling, or the removal of iron oxides, is also practiced in variousways, but the most commonly perior thereto in that the operation can becarried on in a short time, is under close control, and does not resultin the formation of products presenting a waste disposal problem.According to the new method, the articles are pickled by immersion in ahot solution of an acid which is capable of forming an azeotropic orconstant boiling point solution with water. The temperature and strengthof the bath are maintained by vaporizing that solution and directing thehot vapors into the bath where they condense and give up heat thereto.The bath is maintained at a selected level during the operation andoverflow therefrom is collected andv from time to time heated to driveoil! the acid in the form of vapor and to produce a solid residue. Thevapor thus evolved iscollected by appropriate means and recovered forfurther use in the operation while the solid residue is removed from thevaporizer and disposed or in any convenient manner.

The acid preferred for use in the new process is hydrochloric and asolution of that acid containing 20.24% hydrogen chloride has a boilingpoint which, at atmospheric pressure, is about 230 F, and is the maximumfor hydrochloric acid solutions. Any solution of the acid containingless than 20.24% hydrogen chloride will boil at a temperature between212 F, and about 230 F. and the vapor produced will be. relatively richin water. When the acid reaches a concentration of used procedureinvolves immersion of the articles in a sulfuric acid bath. This acid ischeap and non-volatile! but the reaction is slow and theefliciency of asulfuric acid bath is constantly changing in service. This makesagitation of the bath or of the objects desirable, particularly in thepickling of hollow objects, such as tubes. Also, control of theoperation presents considerable dlfllculty, since the percentages ofboth the acid and of the reaction product, ferrous sulphate, present inthe bath, as well as the temperature, are factors influencing thesolution velocity of the bath. In the course of use, the accumulation offerrous sulphate in the bath requires that the bath be renewed and it isa common practice to discard the entire bath when the acid content fallsbelow a level which may be around 5%. The throwingaway of such a bathrepresents a considerable loss of acid and the disposal frequentlypresents a difilcult problem, since the bath would pollute any streaminto which it might be discharged.

The present invention is directed to the pro- Vision of a picklingprocess which avoids the dis advantages of the practice outlined and issu- 20.24% hydrogen chloride, its boiling point is constant andadvantage is taken of this property in pickling according to the newmethod.

For a better understanding of the invention, reference may be had to theaccompanying drawings in which Fig. 1 is a diagrammatic viewillustrating the steps in the complete cleaning operation of which thepickling process of the invention forms a P Fig. 2 is a View, partly inelevation and partly in vertical cross-section, through apparatus usedin the pickling operation: and

Fig. 3 is a view at right angles to Fig. 2 showing a modified form ofthe apparatus on a reduced scale.

The apparatus shown in Fig. 1 is employed in the continuous cleaning ofiron and steel objects which are conveyed through the various vessels inwhich the several operations take place. The objects to be pickled maybe of various forms and the apparatus illustrated is appropriate forcontinuous cleaning of tubes or conduits i0 which are supplied to aconveyor generally designated II. The conveyor is arranged to carry thetubes into and out of the successive tanks and when the tubes requiredegreasing, they are first passed through tank l2 in which thedegreasing operation is performed. It is to be understood, of course,that degreasing may be omitted, if there is no oil or grease on theobjects.

The tank I2 contains an organic solvent, such as trichlorethylene, andis of such length that, as the conveyor passes the tubes through thesolution, the tubes are satisfactorily degreased. From tank i 2, theconveyor carries the tubes over the inclined floor l3 from which thesolvent draining from the tubes may run back into the tank. The solventemployed in degreasing is kept at the appropriate temperature bysuitable means and vapors given oil are collected in hood l2a containingcondensing means. The condensed liquid is returned to the tank and fromtime to time, the solvent bath .is renewed as may be required.

The degreased articles pass from the floor I3 into the pickling tank l4,shown in detail in Figs. 2 and 3. .The tank contains a, solution I5 ofhydrochloric acid, which is maintained at a selected level by anoverflow line I3 leading to a vessel II.

The vessel is also connected to the bottom of the tank by a valvedconnection I8 and has a valved inlet l3. From the bottom of the vessel,9. line 20 containing a valve 2| leads into a vaporizer 22 heated byexternal means illus trated conventionally at 23. Lines 24 and 23connect the line 20 to sources of compressed air and steam,respectively, and flow through lines 24 and 25 is controlled by valves26 and 21 in the respective lines.

The vaporizer is connected by a line 28 and a branch 29 containing avalve 30 to a fractionating column 3|, from the top of which a line 32leads to a condenser 33. The condenser has an air inlet 34 at the topand a bottom outlet 35 connected by a branch 36 containing a valve 31 tovessel 11. Another valved branch 38 from outlet 35 leads to any suitabledischarge point. The line 28 from the vaporizer has another branch 39containing a valve 40, the branch 39 extending into tank l4 andterminating near the bottom thereof within a heat exchanger sectioncomprising horizontal partitions 4|, 42. These partitions are of sucharrangement that vapor discharged from the branch line 39 enters liquidin thespace beneath partition 4| and, as the liquid volume bycondensation of the vapor, the lengthwise of the: tank to the oppositeend and returns between partitions 4| and 42 to enter the main body ofthe bath.

The objects carried through the pickling tank M on the conveyor passover a, floor 43 from which the excess acid may drain off and flow backinto tank i4. Beyond floor 43, the conveyor carries the objects througha tank 44 containing wash water and thence over a floor 45 from whichthe water draining from the objects may return to tank 44. Beyond thefloor 45, the conveyor carries the objects into a tank 46 containing analkaline solution and thence over floor 4'! from which solution'drainingfrom the objects returns to tank 46. At the end of floor 41, the objectsare discharged from the conveyor and transported to the appropriatedelivery point.

A hood 48, exhausted by a fan 49, overlies a portion of floor l3, thetank I4, and the floor 43. The fumes carried away through the hood entera condenser 50 wherein the acid is condensed by cooling means, thecondensate being led away The exhaust gases from the through a pipe 5|.

acid may be considered as condenser are piped into stack 52, which ispreferably outside the building in which the pickling contain less than20.24% hydrogen chloride is introduced into vessel l1 through valveinlet [3. The liquid in the vessel is then allowed to run into thevaporizer 22 through pipe 20 by opening of valve 2!, and after thedesired amount of acid has thus been fed into the vaporizer, valve 2! isshut on and the burner 23 is started.

At this time, valve 30 is open and valve 43 is closed. If the solutionintroduced into the vaporizer contains less than 20.24% hydrogenchloride, heating of the solution results in evolution of vapor which isrelatively rich in water. The vapor is then passed through outlet line28 and branch 29 into the fractionating column 3|; As the water is thelower boiling point member of the binary solution, of which 20.24%hydrochloric the other constituent. water vapor is the diluent from thecolumn and passes through pipe 32 into condenser 33. The distilled watercollecting in the condenser may then be drawn off through outlet 35 andpassed through line 36 into vessel i! for use as make-up water for theacid solution employed in pickling or may be discharged through outlet35 and line 38 and disposed of in the usual way.

A thermometer 53 is mounted in the column 3| near the upper end thereofand when the temperature of the vapor, as indicated by the thermometer,exceeds 212 F., valve 40 is opened and a short time thereafter valve 30is closed and valve 24 is opened successively. These operations resultin the fractionating column being emptied and cause the vapors from thevaporizer to enter the heat exchange section of tank i4 and air fromline 24 to bubble through the liquid in the vaporizer. The vaporscondense in the heat exchange section of tank l4 and give up heat to theliquid in the tank. When the temperature of the liquid'in the tankreaches 230 F., the supply of fuel to the burner 23 is throttled.

The objects are then passed through the bath in tank l4 and the rust andscale thereon are attacked by the acid and removed. The picklingoperation employing hydrochloric acid with the bath maintained at atemperature of about 230 F, is extremely rapid andpickling operationswhich may take hours when carried on by sulfuric acid methods can besuccessfully completed in minutes by the new method.

As the pickling proceeds, the condensation of vapors in tank i4 resultsin overflow of liquid into vessel i1 and this liquid contains ferrouschloride, which is produced by the attack of the acid on the objects andby interaction of the reaction products. The reactions which take placemay be represented by the following equations:

centrated to 20.24% acid, as above described,

and used in the ordinarymanner in the process. As a result of thevaporization of the lower boiling point solution formed in tank I4, asdescribed, the pickling bath tends to maintain its acid concentration at20.24% hydrogen chloride and the solution overflowing from tank I4 intovessel I! thus contains approximately 20.24% hydrogen chloride. Theoverflow is collected in the vessel until a suitable liquid leveltherein is reached and the solution is then allowed to flow intovaporizer 22. Valve 40 is then shut off and valve 30 opened and heat isapplied to the vaporizer as before to drive off vapor which enters thefractionating column. Any excess water present is thus removed and valve-40 is then opened and thereafter valve 30 is closed, so that the vaporproduced from further heating enters the heat exchanger section of thetank as above described.

The liquid in the vaporizer now contains ferrous chloride and probably asmall percentage '6 Any ferrous chloride that may have escaped oxidationin the initial reaction is then converted into ferric chloride asfollows:

The ferric chloride thus produced reacts with steam with formation ofthe oxide and evolution of acid as indicated in the preceding equation.In both reactions above described, the iron oxide resulting from thereaction is indicated as FezOs (ferric oxide), but it is to beunderstood that the type of iron oxide obtained will vary with variationin the pressures and temperatures and the degree of hydration.

In all operations in which a solid residue is to be obtained, it isdesirable to introduce compressed air into the vaporizer just before thelatter is opened to recover the residue. This serves to' expel all vaporfrom the vaporizer and also prevents liquid from the condenser fromsucking back into the vaporizer.

of ferric chloride which has not been reduced.

By continuing the heating of the vaporizer, a solid residue, mainly offerrous chloride crystals, may be obtained and this solid material isthen removed from the vaporizer through a suitable discharge opening.Thereafter liquid from vessel I1 is introduced into the vaporizer andthe sequence of operations, involving elimination of any excess waterthat may be present and the delivery of the vapors of the constantboiling point solution of acid into the tank I4, is repeated.

In some instances, it may be desirable to recover the iron from thepickling bath in the form of ferric chloride rather than ferrouschloride, and this is accomplished by introducin air v through lines 24and 20 into the liquid in vaporizer 2|, while the solution therein isbeing heated to dryness. The air aids in the agitation of the solutionand the reaction is as follows:

After completion of the reaction, the solution may be heated to drynessand the residue, which will be mainly ferric chloride with minor amountsof ferrous chloride remaining unoxidized in the reaction, may beremoved. It is to be understood that there may also be basic chloridespresent in the residue obtained either when ferrous chloride or ferricchloride is the main constituent of the residue.

When the solid residue taken from the vaporizer 22 is either ferrouschloride or ferric chloride, the formation of the residue represents aloss of acid and it may be desirable for economic or other reasons torecover the acid and to produce a residue in the form of iron oxide. Forthis purpose, the ferrous chloride in the solution in the vaporizer mayfirst be converted to ferric chloride, after which both steam and airare introduced into the vaporizer through lines 2, 25, and 20. Theferric chloride then reacts with the steam as follows:

The heating of the solution may be carried on to dryness in thevaporizer, or, if desired, the solution may be concentrated in thevaporizer almost to dryness and then removed in liquid formto be givenits'final treatment at another time or at another place. One reason forthis is that if the vaporizer were to be used only for vaporization, itcould be constructed somewhat like a steam boiler to effect vaporizationrapidly. Such a vaporizer, however, might not be able to stand thehigher temperature required for decomposition of the iron salts. Also,such a vaporizer could probably not be conveniently provided with meansfor removal of solid material. In the practice of the process,therefore, it is immaterial whether the concentration of the solution iscarried to dryness in the vaporizer or elsewhere.

In the foregoing, I have described the introduction of steam and airsimultaneously into the tive in decomposition of the iron salts in theform of a dry or semi-dry residue. For example, if the solution in thevaporizer contains iron mainly in the form of ferrous chloride with aminor proportion of ferric chloride and this solution is heated, theiron salts form lumps deposited at the bottom of the vaporizer. If theshell of any such lump is mainly of ferrous chloride and steam and airare admitted into the vaporizer, the iron salt reacts with the oxygen ofthe air to form ferric chloride which then reacts with the steam to formiron oxide. Any ferric chloride in the lump then exposed to steam andair reacts with the steam and, as the reactions proceed, more of theiron salts within the lump are exposed. The reactions continue until theentire lump has reacted with the steam and air and the salts have beenconverted to iron oxide and hydrogen chloride has been produced. Neithersteam norair alone would produce these results, since the air reactsonly with the ferrous chloride and the steam only with theless-of whichsalt is exposed and in what form.

r The apparatus so far described includes only a single vaporizer andthe pickling operation must be discontinued from time to time forremoval of the iron compounds from the bath. If it is desired to carryon the pickling operation without interruption for indefinite periods,the modified form of apparatus illustrated in Fig. 3 may be used. Thatapparatus includes two vaporlzers 22a and 22b and the discharge lines28a, 28b therefrom have respective branches 29a, 29b, connected togetherand leading into the bottom of the column 3|. The branches 29a, 29b areprovided, respectively, with valves 30a, 30b. Branches 29a, 2% from therespective lines 28a, 281) are also connected together and lead into thetank M, the branches being provided, respectively, with valve 40a, 40b.The Vaporizers 22a, 22b are individually connected to the vessel I! bysuitable lines (not shown), each of which has a valve similar to valve2| and is connected to sources of compressed air and steam throughvalved lines similar to those designated 24 and 25.

In the practice of the new method as above described, the acid issupplied to the pickling tank during the pickling operation in the formof vapor of the constant boiling point solution of the acid and thisprocedure is preferred, since all the advantages of the invention arethereby obtained. However, under some conditions of operation, it may beundesirable or inconvenient to supply the acid in that form, as, forexample, because the pickling is to be carried on at one location andthe reclamation of the pickling bath at another or because it is desiredto employ a pickling bath of a concentration less than that of theconstant boiling point solution. In that situation, suitablemodifications may be made in the pr'acticeof the method to meet localrequlrements and modifications that will be readily apparent includerecovery of the acid in the form of a constant boiling point solutionfrom the spent bath and use of this acid, either diluted vor not, in abath heated extraneously. When that practice is followed, advantage istaken only 01' the reclamation steps of the new method, but theconvenience of carrying on that operation at a part of the plant remotefrom that where the articles are pickled or the desirability of using abath of less concentration than the constant boiling point solution may,in'some instances, oiiset the loss of advantagesavailable when all thesteps of the new method are carried out.

I claim:

1. A method of pickling metallic objects which comprises immersing theobjects in a hot bath of a constant boiling point solution ofhydrochloric acid, withdrawing part of the bath as the picklingoperation proceeds, fractionally distilling the part of the bathwithdrawn to drive 011 water vapor therefrom and produce a constantboiling point solution of hydrochloric acid, discarding the water vapor,heating the said solution to vaporize it and leave a solid residue, andintroducing the vapors produced in the heating operation into the bathto be condensed therein.

2. A method of pickling metallic objects which comprises immersing theobjects in a hot bath of a constant boiling point solution ofhydrochloric acid, withdrawing part of the bath as the picklingoperation proceeds, fractionally distilling the part of the bathwithdrawn to drive off water vapor therefrom and produce a constantboiling point solution of hydrochloric acid, discarding the water vapor,heating the constant boiling point solution to vaporize it and to leavea solid residue, introducing air into the solution during the heatingthereof, and introducing the vapors of the constant boiling pointsolution into the bath to be condensed therein to maintain thetemperature and eifectiveness of said bath.

3. A method of pickling metallic objects which comprises immersing the.objects in a hot bath of a constant boiling point solution ofhydrochloric acid, withdrawing part of the bath as the picklingproceeds, fractionally distillin the part of the bath withdrawn to driveoff water vapor therefrom and produce a constant boiling point solutionof hydrochloric acid, discarding the water vapor, heating the solutionto drive 011' vapors therefrom and to concentrate it, introducing thevapors of said solution into the bath to be condensed therein to heatthe bath and maintain the eifectiveness thereof, discontinuing theheating of the concentrated solution, and, in a subsequent operation,heating the concen trated solution to dryness.

JAMES W. HUDSON.

REFERENCES CITED UNITED STATES PATENTS Number Name Date 389,058 ColeSept. 4, 1888 1,272,917 Cooke July 16, 1918 1,146,071 Hoifman July 13,1915 1,853,330 Barstow Apr. 12, 1932 2,017,773 Smith Oct. 15, 19352,123,434 Paulson July 12, 1938 2,193,051 Wood Mar. 12, 1940 2,228,135Franwick Jan. 7, 1941 2,235,658 Waterman Mar. 18, 1941 FOREIGN PATENTSNumber Country Date 273,307 Great Britain Apr. 5, 1928

